Process for brominating butadiene/vinyl aromatic copolymers

Abstract

Butadiene copolymers are brominated in solution using bromine as the brominating agent. The bromination is conducted in the presence of an excess of an aliphatic alcohol, relative to the amount of bromine that is used. The bromination proceeds rapidly at mild conditions, and is selective in that only aliphatic carbon-carbon double bonds are brominated, and unwanted bromination at tertiary carbon atoms is largely or completely avoided. The resulting brominated polymers are characterized in having very good thermal stability.

What is claimed is: 
     
       1. A process of preparing a brominated butadiene copolymer, the process comprising
 (a) contacting a solution of a starting copolymer of butadiene and at least one vinyl aromatic monomer in a non-oxygenated solvent with Br 2  in an amount of from 0.5 to 1.5 equivalents per equivalent of aliphatic carbon-carbon double bonds in the starting copolymer to form a reaction solution, said contact being in the presence of from 0.75 to 3.5 moles of an aliphatic alcohol per mole of Br 2 ; and 
 (b) maintaining the reaction solution under reaction conditions for a period of time sufficient to brominate more than 25 percent of aliphatic double bonds contained in the starting copolymer. 
 
     
     
       2. The process of  claim 1 , wherein the aliphatic alcohol has a secondary or tertiary hydroxyl group. 
     
     
       3. The process of  claim 1  wherein the aliphatic alcohol has from 1 to 8 carbon atoms. 
     
     
       4. The process of  claim 3 , wherein the aliphatic alcohol is 2-propanol, 2-butanol, t-butanol or 2-pentanol. 
     
     
       5. The process of  claim 4 , wherein in step a), from 0.8 to 1.2 equivalents of bromine are contacted with the solution of starting copolymer per equivalent of aliphatic carbon-carbon double bonds in the starting copolymer. 
     
     
       6. The process of  claim 1 , wherein the starting polymer contains 1,2-butadiene units, and the 1,2-butadiene units constitute at least 50% of the butadiene units in the starting polymer. 
     
     
       7. The process of  claim 6 , wherein the starting polymer has a weight average molecular weight, by GPC relative to a polystyrene standard, of from 50,000 to 175,000. 
     
     
       8. The process of  claim 1 , wherein steps a) and b) are conducted at a temperature of from −20 to 85° C. 
     
     
       9. The process of  claim 8  wherein the starting polymer is a block copolymer of styrene and butadiene. 
     
     
       10. The process of  claim 1  which is operated continuously. 
     
     
       11. The process of  claim 1 , further comprising contacting the brominated copolymer with an alkali metal alkoxide. 
     
     
       12. The process of  claim 10 , further comprising contacting the brominated copolymer with an alkali metal alkoxide, wherein from 0.05 to 0.20 mole of the alkali metal alkoxide is contacted with the brominated copolymer per mole of repeating brominated or unbrominated butadiene units in the copolymer.